The instant invention generally relates to sensors and systems for measuring weight and more particularly to a weight sensor for measuring the weight of occupants and other objects in a motor vehicle seat such as useful determining occupant seating conditions for controlling a vehicle safety restraint system.
A vehicle may contain automatic safety restraint actuators that are activated responsive to a vehicle crash for purposes of mitigating occupant injury. Examples of such restraint actuators include air bags, seat belt pretensioners, and deployable knee bolsters.
One objective of an automatic safety restraint system is to mitigate occupant injury, thereby not causing more injury with the automatic restraint system than would be caused by the crash had the automatic restraint system not been activated. Notwithstanding the protective benefit of these automatic safety restraint actuators, there is generally both a risk and a cost associated with the deployment thereof. Generally, it is desirable to only activate automatic safety restraint actuators when needed to mitigate injury because of the expense of replacing the associated components of the safety restraint system, and because of the potential for such activations to harm occupants. This is particularly true of air bag restraint systems, wherein occupants too close to the air bag at the time of deploymentxe2x80x94i.e. out-of-position occupantsxe2x80x94are vulnerable to injury or death from the deploying air bag even when the associated vehicle crash is relatively mild. Moreover, occupants who are of small stature or with weak constitution, such as children, small adults or people with frail bones are particularly vulnerable to injury induced by the air bag inflator. Furthermore, infants properly secured in a normally positioned rear facing infant seat (RFIS) in proximity to a front seat passenger-side air bag are also vulnerable to injury or death from the deploying air bag because of the close proximity of the infant seat""s rear surface to the air bag inflator module.
Air bag inflators are designed with a given restraint capacity, as for example, the capacity to protect an unbelted normally seated fiftieth percentile occupant when subjected to a 30 MPH barrier equivalent crash, which results in associated energy and power levels which can be injurious to out-of-position occupants. While relatively infrequent, cases of injury or death caused by air bag inflators in crashes for which the occupants would have otherwise survived relatively unharmed have provided the impetus to reduce or eliminate the potential for air bag inflators to injure the occupants which they are intended to protect.
One technique for mitigating injury to occupants by the air bag inflator is to reduce the power and energy levels of the associated air bag inflator, for example by reducing the amount of gas generant in the air bag inflator, or the inflation rate thereof. This reduces the risk of harm to occupants by the air bag inflator while simultaneously reducing the restraint capacity of the air bag inflator, which places occupants a greater risk for injury when exposed to higher severity crashes.
Another technique for mitigating injury to occupants by the air bag inflator is to control the rate of inflation or the capacity of the inflator responsive to a measure of the severity of the crash. However, the risk of injury to such occupants would not be mitigated under the conditions of higher crash severity when the inflator is intentionally made aggressive in order to provide sufficient restraint for normally positioned occupants.
Yet another technique for mitigating injury to occupants by the air bag inflator is to control the activation of the air bag inflator responsive to the presence, position, and size of the occupant, or to the severity of the crash. For example, the air bag inflator can be disabled if the occupant weight is below a given threshold. Moreover, the inflation capacity can be adjusted by controlling the number of inflation stages of a multi-stage inflator that are activated. Furthermore, the inflation power can be adjusted by controlling the time delay between the firings of respective stages of a multi-stage inflator.
One measure of restraint capacity of an air bag inflator is the amount of occupant kinetic energy that can be absorbed by the associated air bag system, whereby when the occupant collides with the gas filled air bag, the kinetic energy of the occupant is converted to potential energy via the pressurization of the air bag, and this potential energy is dissipated by venting pressurized gases from the air bag. As a vehicle in a crash is decelerated, the velocity of an unrestrained occupant relative to the vehicle increases. Preferably, the occupant restraint process is commenced early in the crash event so as to limit the amount of occupant kinetic energy that must be absorbed and thereby minimize the associated restraint forces and accelerations of and loads within the occupant. If the occupant were a simple inertial mass without friction relative to the vehicle, the kinetic energy of the occupant would be given by xc2xd Mxc2x7V2, where M is the mass of the occupant and V is the occupant velocity relative to the vehicle. If a real occupant were represented by an interconnected set of bodies, some of which have friction relative to the vehicle, each body of which may have differing velocities relative the vehicle, the above equation would apply to the motion of the center of gravity of the occupant. Regardless of the representation, occupants of larger mass will have a larger kinetic energy for the same velocity relative to the vehicle. Therefore, an occupant weight sensor is useful in an air bag system with variable restraint capacity to enable the restraint capacity to be preferentially adapted to the weight, or mass, of the occupant.
Except for some cases of oblique or side-impact crashes, it is generally desirable to not activate an automatic safety restraint actuator if an associated occupant is not present because of the otherwise unnecessary costs and inconveniences associated with the replacement of a deployed air bag inflation system. Occupant presence can be detected by a seat weight sensor adapted to provide either a continuous measure of occupant weight or to provide a binary indication if the occupant weight is either above or below a specified weight threshold.
Known seat weight sensors comprise one or more pads employing force sensitive resistive (FSR) films. These arrangements are typically used as weight threshold systems to disable a passenger air bag when the seat is empty. Load cells attached to the seat mounting posts have also been used in research applications. Mechanisms that use string based potentiometers to measure downward seat displacement have also been investigated.
Such known arrangements suffer from several drawbacks. First, variable resistance force sensors have limited sensitivity and in some situations are not sensitive enough to put directly under a seat pad while still achieving the desired response. Second, the threshold weight system provides only very limited information. For example, such arrangements provide no indication as to the size of an occupant. Third, the resistance values of known variable force resistor change with temperature, and are subject to drift over time with a constant load on the sensor.
Furthermore, other known sensing arrangements do not otherwise provide suitable results. For example, the use of load cells is prohibitively expensive for large-scale commercial applications. Strain gauges of any type may be impractical because of the difficulty in applying them to the strained material. Finally, mechanical string potentiometer based weight sensors are complex, and subject to failure from stretching of the string.
The prior art also teaches the use of seat weight sensors outside the automotive environment, for example as a means for disabling the activation of either a boat or an industrial machine if the operator is not properly seated, or for weighing a person seated on an exercise bike. These devices employ pneumatic bladders located in the seat, whereby the pressure within the bladder is used to either activate a threshold switch or to provide a continuous indication of occupant weight.
One problem with prior art pneumatic sensors, particularly when applied to the automotive environment, is their sensitivity to environmental conditions, particularly to ambient temperature and pressure. A seat weight sensor in an automotive environment must function reliably and accurately over a wide range of temperatures and pressures which can cause significant errors.
The prior art also teaches the use of hydraulic load cells, wherein the weight to be measured acts upon a piston element of known area, whereby the measured weight is found by multiplying a measured pressure times the known area. One problem with hydraulic load cells in the automotive environment, particularly in a seat, is that the effects of load cell orientation on hydraulic head can introduce load measurement errors.
The instant invention overcomes the above-noted problems by providing a seat weight sensor which incorporates a fluid containing bladder placed in series with the load path in the seat, whereby a load applied to and distributed across the bladder increases the pressure of the fluid therein. The pressure of the fluid is measured by a pressure sensor and is substantially proportional to the magnitude of the applied load, and substantially inversely proportional to the supported area of the bladder. The instant invention also incorporates a means for distributing the applied load across the area of the fluid containing bladder so as to prevent a concentrated load from compressing the top and bottom surfaces of the bladder against one another and thereby creating an alternate load path which does not cause an associated pressurization of the fluid. The output signal is substantially linear with respect to weight provided that 1) the weight is distributed over a sufficient area so that the bladder does not bottom out, 2) the height of the bladder is sufficiently small relative to the base dimensions so that the effect of loading on the support area is relatively small. Preferably, the amount of fluid in the bladder should be less than the capacity of the bladder when the bladder is unloaded. Otherwise, the fluid in the bladder can be pressurized by increasing temperature or decreasing ambient pressure which results in associated load measurement errors.
The bladder may incorporate either a liquid or a gas as the sensing fluid. A gaseous sensing fluid is prone to expansion and contraction resulting from changes in ambient temperature and pressure relative to the conditions under which the bladder was initially filled. A gaseous fluid is also more prone to leakage and to localized collapse of the top and bottom surfaces of the bladder under the influence of a concentrated load. When located in the seat under a cushion, the cushion can provide an effective distribution of the loads applied to the seat. A sheet of semi-rigid material can also be used to distribute load to the bladder, particularly the reaction forces from the seat springs if the bladder is located thereon.
The bladder may incorporate internal seams which secure the top and bottom surfaces of the bladder to one another within the periphery of the bladder without disrupting the fluid communication within the bladder. These seams prevent the bladder from bulging in the center when the fluid expands due to temperature or pressure effects. Such bulging is detrimental to seating comfort. The seams also assist reducing the overall thickness of the bladder and in conserving the necessary amount of sensing fluid, which reduces cost when liquids such as silicone based fluids are used. The internal seams are also effective for modifying the sensitivity of the bladder. For example, a bladder may be more sensitive to central loads than to distal loads as might result when a portion of the applied load is carried by a portion of the seat cushion which is not in series with the load bladder load path. In this case, selective zones within the bladder, for example near the center, may be isolated from the sensing fluid by a closed path seam such that a load applied thereto is not sensed by the fluid within the bladder.
The bladder may be constructed from several sheets of fabric, such as nylon, coated with a sealably weldable coating, such as polyurethane which can be RF (radio frequency) welded. A coating can be applied to the outside of the bladder to increase the membrane stiffness thereof and thereby facilitate the distribution of applied loads.
The instant invention integrates pressure over the entire loading area of the seat, thereby producing a consistent output signal that is relatively insensitive to the associated load distribution. The instant invention is relatively flexible, and when installed under the seat cushion does not interfere with seating comfort. Furthermore, this installation is relatively easy, thereby minimizing the impact on the overall manufacturing process of the seat/vehicle.
Accordingly, one object of the instant invention is to provide an improved seat weight sensor which provides a consistent and accurate measure of the seat loading independent of the location of the source of weight on the seat.
A further object of the instant invention is to provide an improved seat weight sensor which provides a consistent and accurate measure of the seat loading independent of the size and distribution of the source of weight on the seat.
A yet further object of the instant invention is to provide an improved seat weight sensor which provides a consistent and accurate measure of the seat loading independent of the amount of weight on the seat.
A yet further object of the instant invention is to provide an improved seat weight sensor which operates under a wide range of ambient temperature and pressure conditions.
A yet further object of the instant invention is to provide an improved seat weight sensor which can distinguish between a rear facing infant seat, for which an air bag system is preferably not deployed, and other occupants for which an air bag system is preferably deployed in the event of a crash of sufficient severity.
A yet further object of the instant invention is to provide an improved seat weight sensor which can be incorporated into an intelligent safety restraint system for which the preferable mode of the activation of a controllable occupant restraint system is dependent upon the weight of the occupant.
A yet further object of the instant invention is to provide an improved seat weight sensor which does not interfere with occupant comfort.
A yet further object of the instant invention is to provide an improved seat weight sensor which is insensitive to the orientation of the seat.
A yet further object of the instant invention is to provide an improved seat weight sensor which is inexpensive to produce.
In accordance with these objectives, one feature of the instant invention is a fluid containing bladder mounted in the base of the seat.
Another feature of the instant invention is a pressure sensor operatively coupled to the fluid containing bladder for measuring the pressure therein.
Yet another feature of the instant invention is a differential pressure sensor operatively coupled to the fluid containing bladder for measuring the pressure therein relative to local atmospheric pressure.
Yet another feature of the instant invention is the incorporation of a gas as the fluid in the fluid containing bladder.
Yet another feature of the instant invention is the incorporation of a liquid as the fluid in the fluid containing bladder.
Yet another feature of the instant invention is the incorporation of a means for distributing load across the load bearing surface of the bladder.
Yet another feature of the instant invention is that the volume of fluid in the fluid containing bladder is such that the volume of the bladder in an unloaded state is less than the maximum volume of the bladder over the range of environmental operating conditions.
The specific features of the instant invention provide a number of associated advantages. One advantage of the instant invention with respect to the prior art is that the fluid containing bladder is responsive to loads over a large area of the seat without regards to the distribution or amount of loading.
Another advantage of the instant invention is that the output signal is inherently relatively linear which simplifies signal analysis.
Yet another advantage of the instant invention is that the seat weight sensor thereof can enable a rear facing infant seat for which the air bag system is preferably not deployed to be distinguished from an occupant for which the air bag system is preferably deployed.
Yet another advantage of the instant invention is that the seat weight sensor thereof is sufficiently robust, reliable and accurate to enable associated occupant weight dependent control of a controllable occupant restraint system.
Yet another advantage of the instant invention is that the seat weight sensor thereof is relatively inexpensive to produce.
Accordingly, the instant invention provides an improved seat weight sensor which is relatively insensitive to the effects of ambient temperature and pressure; which is simple in construction and relatively robust and reliable in operation; which can be readily incorporated into an automotive seat without interfering with occupant comfort; and which can be produced relatively inexpensively.
The instant invention will be more fully understood after reading the following detailed description of the preferred embodiment with reference to the accompanying drawings. While this description will illustrate the application of the instant invention in an automotive safety restraint system, it will be understood by one with ordinary skill in the art that the instant invention can also be applied to other systems for weighing objects.